Hi, if I have Houdini Indie installed in one computer, but, I need to replace my computer, can I re-install the license in the new one? - Note: I'm using a Mac, and my new computer will be Windows. Cheers .

I think you can return the licenses, and then re-redeem it in another machine. I think there's a limit of 2 returns. See here:
https://www.sidefx.com/index.php?option=com_content&task=view&id=1208#return [sidefx.com]
If unsure, contact support.